1. Field of the Invention
This invention relates to an optical recording medium such as a read only optical recording disk and an optical recording disk.
2. Prior Art
Optical information media such as read-only optical disks and optical recording disks have been required to have a higher capacity by increasing the recording density for the purpose of recording and storing an enormous amount of information as in the case of motion picture information. Extensive efforts have been dedicated to the research and development of the recording at a higher density to meet such request.
Under such situation, one proposal has been use of a smaller laser beam spot with a reduced diameter in the recording and reading as in the case of DVD (Digital Versatile Disk) by reducing the wavelength used in the recording/reading and increasing the numerical aperture (NA) of the objective lens of the recording/reading optical system. When the DVD is compared to CD, the DVD has realized a recording capacity (of 4.7 GB/side) which is 6 to 8 times larger than that of the CD by reducing the recording/reading wavelength from 780 nm to 650 nm and by increasing the NA from 0.45 to 0.6.
Use of a higher NA, however, invites decrease of tilt margin. Tilt margin is tolerance for the tilting of the optical information medium in relation to the optical system, and the tilt margin is determined by the NA. When the recording/reading wavelength is xcex, and the transparent substrate through which the medium is irradiated with the writing/reading has a thickness t, the tilt margin is proportional to
xcex/(txc2x7NA3) 
Tilting of the optical recording medium at an angle to the laser beam, namely, occurrence of the tilt results in the generation of wave front aberration (coma aberration). When the substrate has a refractive index of n and a tilt angle of xcex8, the wave front aberration coefficient is given by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2 xcex8)xe2x88x925/2 
These relations indicate that decrease in the thickness t of the substrate is effective when the tilt margin is to be increased with simultaneous suppression the generation of the coma aberration. As a matter of fact, tilt margin is ensured in the case of DVD by reducing the thickness of the substrate to about half (about 0.6 mm) of the thickness of the CD (about 1.2 mm). In the meanwhile, margin of the thickness unevenness of the substrate is given by
xcex/NA4 
When the substrate has an uneven thickness, such uneven thickness further results in wave front aberration (spherical aberration). When the substrate has a thickness unevenness of At, the spherical aberration coefficient is given by
{(n2xe2x88x921)/8n3}xc2x7NA4xc2x7xcex94t 
As indicated by these relations, the thickness unevenness of the substrate should be reduced in order to reduce the spherical aberration associated with the increase in the NA. For example, in the case of DVD, xcex94t is suppressed to xc2x130 xcexcm compared to that of xc2x1100 xcexcm in the CD.
A structure enabling further decrease in the substrate thickness has been proposed in order to realize high quality motion picture recording for a longer period. In this structure, a substrate having normal thickness is used as a supporting substrate for ensuring rigidity of the medium, and the pits and the recording layer are formed on its surface, and a light-transmitting layer in the form of a thin substrate having a thickness of about 0.1 mm is formed on the recording layer. The medium is irradiated with the recording/reading beam through this light-transmitting layer. This structure enables drastic reduction in the thickness of the substrate, and high density recording by the use of a higher NA is thereby enabled. A medium having such structure is described, for example, in Japanese Patent Application Laid-open Nos. (JP-A) 320859/1998 and 120613/1999.
The medium described in JP-A 320859/1998 is a magneto-optical recording medium, and this magneto-optical recording medium has a structure wherein a metal reflective layer, a first dielectric layer, a magneto-optical recording layer, a second dielectric layer, and a light-transmitting layer are disposed on the substrate in this order. In JP-A 320859/1998, surface roughness of the metal reflective layer at the interface between the dielectric layer and the metal reflective layer is reduced to the level of less than 8.0 nm based on the view that increase in the noise of the read-out signal is induced by the excessively large surface roughness of the metal reflective layer formed by sputtering. In JP-A 320859/1998, an aluminum-containing material, and preferably, a material containing aluminum in admixture with at least one member selected from Fe, Cr, Ti and Si, or Au or Ag is used for constituting the metal reflective layer, and ion beam sputtering or magnetron sputtering is employed for the layer formation.
The medium described in JP-A 120613/1999 is a phase change optical recording medium wherein the medium is formed by disposing a reflective layer, a phase change recording layer, and a light-transmitting layer on the substrate in this order. This medium also reduces the surface irregularity of the reflective layer by adopting the reflective layer of particular composition. There is stated in JP-A 120613/1999 that xe2x80x9cmorphology of the boundary reflecting the grain size determined by the crystallinity of the reflective layer and the composition of the reflective layerxe2x80x9d, and therefore, it is understood that the surface roughness of the metal reflective layer is reduced in JP-A 120613/1999 by reducing the grain size.
However, in view of optimizing recording sensitivity and reflectivity in the actual medium design, it is not practical that a limitation is set on the material used for the metal reflective layer in order to reduce the surface irregularity.
In addition, it has been found in the investigation of the inventors of the present invention that, in the case of an optical recording medium wherein heat mode recording is employed as in the case of a magneto-optical recording medium or a phase change optical recording medium, an excessive reduction in the grain size of the reflective layer results in various problems in the recording properties of the medium.
An object of the present invention is to reduce the noise of the read-out signal in an optical information medium comprising a supporting substrate and a reflective layer disposed on the supporting substrate wherein the reading beam irradiates the medium from the side where the reflective layer is formed. Another object of the present invention is to reduce the noise of the read-out signal with no adverse effects on the recording properties in an optical information medium comprising a supporting substrate, and a reflective layer and a recording layer disposed on the supporting substrate in this order wherein the recording beam irradiates the medium from the side where the reflective layer is formed.
Such objects are attained by the present invention as described in (1) to (3), below.
(1) An optical information medium comprising a supporting substrate and a reflective layer disposed on the supporting substrate, wherein said reflective layer has a crystallite size of up to 30 nm, and the medium is used such that the reading beam irradiates the medium from the side where the reflective layer is formed.
(2) The optical information medium according to the above (1) wherein a recording layer is formed on the reflective layer, and the medium is used such that the recording beam irradiates the medium from the side where the reflective layer is formed.
(3) The optical information medium according to the above (2) wherein the reflective layer has an average grain size of at least 20 nm.